Hot rolled steel sheets and method of producing the same

ABSTRACT

After a starting material of steel comprising C: 0.001-0.20 wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %, S: not more than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than 0.020 wt % and the balance being Fe and inevitable impurities is heated above AC 3  point, rough rolling is completed within a temperature range of (Ar 3  point+100° C.)-(Ar 3  point+50° C.), and super-high pressure descaling is carried out under conditions satisfying a jetting pressure of not less than 25 kgf/cm 2  and a liquid quantity density of not less than 0.002 liter/cm 2 , and subsequently finish rolling at a rolling reduction of not less than 80% above Ar 3  point of a rolling completion temperature is started in 5 seconds and coiling is carried out below 700° C., whereby hot rolled steel sheets having a surface roughness Ra of not more than 0.8 μm and an average scale thickness of not more than 4 μm is produced and the adhesion property in case of subjecting to shaping at a mill scale state and the pickling efficiency in case of applying to pickling are improved.

TECHNICAL FIELD

The present invention relates to hot rolled steel sheets, particularlysteel sheets as-rolled alone or further cold rolled and a method ofproducing the same, and more particularly to a hot rolled steel sheethaving such a thin scale that the peeling of scale is less in theworking as a mill scale (as-rolled), while the pickling efficiency isgood in applications after the pickling and a surface roughness Ra isnot more than 0.8 μm and an average scale thickness is not more than 4μm, and a method of producing the same.

BACKGROUND ART

In general, the hot rolled steel sheets are produced by hot rolling aslab of steel obtained through a continuous casting method or a bloomingmethod. In a surface layer of the thus obtained hot rolled steel sheetis created so-called secondary scale produced during the hot rolling andcomprised of three layers of FeO--Fe₃ O₄ --Fe₂ O₃ having a thickness ofabout 5 μm˜15 μm.

When the secondary scale created on the surface of the hot rolled steelsheet is subjected to a shaping work at a mill scale state (at a stateof holding the mill scale on the surface of the hot rolled steel sheet),a part of the scale is peeled off to contaminate the working line, orthe peeled scale induces a surface defect of a product after the work asan indentation flaw. For this end, a slightly light degree of theworking has hitherto been conducted to the hot rolled steel sheet havingthe mill scale.

Under the above circumstance, when such the hot rolled steel sheet issubjected to a work at a large strain amount or is used as a startingmaterial for cold rolled steel sheet, it is necessary to attempt theremoval of the scale through a pickling step. Even in this case, when acoiling temperature after the hot rolling is rendered into a hightemperature above 550° C. from the reason of the material properties inthe conventional technique, there are problems that the scale existingon the edge of the steel sheet thickly grows, and the transformationfrom FeO to Fe₃ O₄ +Fe is caused to densify the scale and hence thepickling efficiency is lowered to considerably increase the load to thework line.

In order to mitigate the aforementioned problems exerting on the scale,therefore, there have been attempted some efforts for thinning thescale.

For example, JP-B-6-104853 discloses a method wherein steel containingSi: 0.02-0.2% and Cr: 0.02-0.2% is soaked to 1150° C. and the rolling ata rolling reduction of not less than 90% is started at not higher than1000° C. and terminated at not higher than 860° C. and then the coilingis carried out at not higher than 500° C.

As a method of removing scale in the course of the hot rolling, forexample, JP-A-4-238620 discloses a method wherein when hot rolled steelsheets are manufactured by subjecting a kind of steels creating hardlypeelable scale to hot rolling, descaling is carried out by jetting ahigh-pressure spraying water onto the surface of the steel sheet at ajetting pressure per unit area of 20-40 g/mm² and a flowing quantity of0.1-0.2 liter/min·mm² prior to a finish rolling.

However, there is a problem that the above method of JP-B-6-104853 isnot applicable to a kind of steel requiring a coiling temperature ofhigher than 500° C. from a viewpoint of the material because itrestricts the coiling temperature after the hot rolling to not higherthan 500° C.

In the method of JP-A-4-238620, a greater part of scale is removed, butthere is a problem that in case of a kind of steel containing a greatamount of Si, scale of a structure entering into matrix is created andcan not be removed and hence scale flaw called as red scale is causedafter the rolling. And also, this method has a problem that it is notnecessarily enough to provide the thin scale.

Moreover, only the steel sheets having a surface roughness Ra of about1-3 μm are obtained by these conventional techniques, so that when theyare subjected to forming work at the mill scale state, sufficientformability (slidability) and adhesion property are not obtained, whilewhen they are used after the pickling, there is a problem that thepickling property is obstructed.

It is, therefore, an object of the invention to provide hot rolled steelsheets without the above-described problems involved in hot rolled steelsheet scale and a method of producing the same.

It is another object of the invention to provide a method ofadvantageously producing a thin-scale hot rolled steel sheet by applyinga super-high pressure descaling.

It is a further object of the invention to provide hot rolled steelsheets having a thin scale at an average scale thickness of not morethan 4 μm and a surface roughness (Ra) of not more than 0.8 μm withoutcausing troubles on workability and pickling efficiency as a mill scalestate even if the coiling temperature is high or if a greater amount ofSi is included as well as a method of producing the same.

DISCLOSURE OF THE INVENTION

The inventors have mainly noticed the descaling conditions prior tofinish rolling in order to achieve the above objects and made variousstudies and found that the scale properties of the steel sheet surfacecan largely be improved by applying super-high pressure descaling, whichhas never been used in the conventional technique, in order to realizethe objects, and as a result the invention has been accomplished. Thatis,

(1) The invention is a hot rolled steel sheet comprising C: 0.001-0.20wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %,S: not more than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than0.020 wt % and the balance being Fe and inevitable impurities, andhaving a surface average scale thickness of not more than 4 μm and asurface roughness (Ra) of not more than 0.8 μm.

(2) The invention is a hot rolled steel sheet comprising C: 0.001-0.20wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %,S: not more than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than0.020 wt %, one or two of Ti: not more than 0.10 wt % and Nb: not morethan 0.10 wt % and the balance being Fe and inevitable impurities, andhaving a surface average scale thickness of not more than 4 μm and asurface roughness (Ra) of not more than 0.8 μm.

(3) The invention is a hot rolled steel sheet comprising C: 0.001-0.20wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %,S: not more than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than0.020 wt %, B: not more than 0.0100 wt % and the balance being Fe andinevitable impurities, and having a surface average scale thickness ofnot more than 4 μm and a surface roughness (Ra) of not more than 0.8 μm.

(4) The invention is a hot rolled steel sheet comprising C: 0.001-0.20wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %,S: not more than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than0.020 wt %, one or two of Ti: not more than 0.10 wt % and Nb: not morethan 0.10 wt %, B: not more than 0.0100 wt % and the balance being Feand inevitable impurities, and having a surface average scale thicknessof not more than 4 μm and a surface roughness (Ra) of not more than 0.8μm.

(5) The invention is a method of producing a hot rolled steel sheet,which comprises heating a starting material of steel comprising C:0.001-0.20 wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than0.05 wt %, S: not more than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: notmore than 0.020 wt % and the balance being Fe and inevitable impuritiesto not lower than Ac₃ point, completing rough rolling within atemperature range of (Ar₃ point+100° C.)˜(Ar₃ point+50° C.), conductingsuper-high pressure descaling under conditions satisfying a jettingpressure of not less than 25 kgf/cm² and a liquid quantity density ofnot less than 0.002 liter/cm², starting finish rolling at a rollingreduction of not less than 80% above Ar₃ point of rolling completetemperature within 5 seconds and coiling up below 700° C.

(6) The invention is a method of producing a hot rolled steel sheet,which comprises heating a starting material of steel comprising C:0.001-0.20 wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than0.05 wt %, S: not more than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: notmore than 0.020 wt %, one or more of Ti: not more than 0.10 wt %, Nb:not more than 0.10 wt % and B: not more than 0.0100 wt % and the balancebeing Fe and inevitable impurities to not lower than Ac₃ point,completing rough rolling within a temperature range of (Ar₃ point+100°C.)˜(Ar₃ point+50° C.), conducting super-high pressure descaling underconditions satisfying a jetting pressure of not less than 25 kgf/cm² anda liquid quantity density of not less than 0.002 liter/cm², startingfinish rolling at a rolling reduction of not less than 80% above Ar₃point of rolling complete temperature within 5 seconds and coiling upbelow 700° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation among jetting pressure, wateramount and average scale thickness of hot rolled sheet.

FIG. 2 is a graph showing a relation between lapse time starting finishrolling after descaling and average scale thickness of hot rolled sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferable conditions for carrying out the invention will be describedbelow.

(1) As to steel components

C: 0.001-0.20 wt %

C is an element required for ensuring the strength. When the amount isless than 0.001 wt %, there is no effect of ensuring the strength, whilewhen it exceeds 0.20 wt %, CO gas is generated at a boundary betweenscale and matrix to cause the peeling of scale in the course of therolling resulting in scale flaw, so that the amount is 0.001-0.20 wt %,preferably 0.001-0.10 wt %.

Si: 0.01-0.50 wt %

Si is used for deoxidation and is an element for improving the strength.When the amount is less than 0.01 wt %, there is no effect, while whenit exceeds 0.50 wt %, scale flaw such as red scale is apt to be caused,so that the amount is 0.01-0.50 wt %, preferably 0.01-0.2 wt %.

Mn: 0.05-2.0 wt %

Mn renders solid-soluted S resulting in the brittleness at hot work intoharmless MnS and is an element effective for the improvement of thestrength. When the amount is less than 0.05 wt %, there is no effect,while when it exceeds 2.0 wt %, the toughness is lowered, so that theamount is 0.05-2.0 wt %, preferably 0.05-1.0 wt %.

P: not more than 0.05 wt %

P badly exerts upon the grain boundary embrittlement and is desirable todecrease the amount as far as possible. When the P content exceeds 0.05wt %, the bad influence is apt to be caused, so that it is not more than0.05 wt %, preferably not more than 0.01 wt %. Moreover, when the amountis decreased to not more than 0.001 wt % under the present refiningtechnique, the steel-making cost considerably increases, so that thelower limit is 0.001 wt % in view of economy.

S: not more than 0.05 wt %

S is an element degrading the hot workability and toughness. When the Scontent exceeds 0.05 wt %, the bad influence becomes conspicuous, it isnot more than 0.05 wt %, preferably not more than 0.01 wt %. Moreover,when the amount is decreased to not more than 0.001 wt % under thepresent refining technique, the steel-making cost considerablyincreases, so that the lower limit is 0.001 wt % in view of economy.

sol.Al: 0.01-0.10 wt %

Al is an element added as a deoxidizing agent, if necessary. When thecontent is less than 0.01 wt % as sol.Al, there is no effect, while whenit exceeds 0.10 wt %, not only the cost rises up but also the steelsheet is embrittled, so that the amount is 0.01-0.1 wt %. Moreover, itis preferably 0.04-0.1 wt % from a viewpoint of the cost performance.

N: not more than 0.020 wt %

N may be utilized for the strengthening by positive addition, but is anelement embrittling the steel sheet when it is excessively includedexceeding 0.020 wt %. Therefore, it is added within a range of not morethan 0.020 wt %, if necessary. Particularly, if the strengthening is notrequired, the amount is preferably not more than 0.01 wt %. Moreover,when the amount is decreased to not more than 0.001 wt % under thepresent refining technique, the steel-making cost considerablyincreases, so that the lower limit is 0.001 wt % in view of economy.

Ti: not more than 0.10 wt %, Nb: not more than 0.10 wt %

Ti and Nb are elements forming carbon-nitrides, and are added forimproving elongation and r-value through the reduction of solid solutionC,N and increasing the strength through fine carbonitride. When eachamount added exceeds 0.10 wt %, the peeling of scale is caused to bringabout the occurrence of scale flaw, so that they are not more than 0.10wt %. Moreover, the preferable addition amount is 0.01-0.06 wt %.

S: not more than 0.0100 wt %

B controls the grain boundary embrittlement produced when the totalamount of solid solution C and N is decreased to not more than 0.0005 wt% and has an effect of enhancing the hardenability, and is an element inaccordance with the necessity. However, when it is added in an amountexceeding 0.0100 wt %, the steel is hardened to cause embrittlement, sothat the amount is not more than 0.0100 wt %. Moreover, the preferableaddition amount is 0.0005-0.0030 wt %.

(2) As to production conditions;

a. The sufficient heating of the steel material before the hot rollingis sufficient to attain the complete solution, so that the heating maybe carried out above Ac₃ point. Concretely, the usual slab heatingtemperature range of 1050°-1300° C. is suitable.

b. Following to the above heating, there are carried out hot roughrolling, descaling with a super-high pressure water and hot finishrolling.

Among these steps, the particularly important features in the inventionincluding limited reasons thereof will be described below.

At first, the reason why the rough rolling is completed at (Ar₃point+100° C.)-(Ar₃ point+50° C.) is due to the fact that the steelsurface is partly transformed from γ to α in the subsequent descaling tosoften the surface and provide a smooth surface and hence a surfaceroughness of Ra≦0.8 μm may be attained. That is, when the completiontemperature of the rough rolling exceeds Ar₃ point+100° C., the surfacelayer is subjected to descaling at a state of γ region, so that thestrength is high and the surface roughness of Ra: not more than 0.8 μmis not obtained. While, when it is lower than Ar₃ point+50° C.,α-transformation proceeds in the descaling and the strength ratherincreases and hence the desired roughness can not be attained likewisethe above.

In the thus obtained thin-scale steel sheet having a low surfaceroughness, it is possible to conduct the descaling in a very short timein the pickling and also the concentration of stress is controlled inthe light plastic deformation to provide a very excellent adhesionproperty.

After the above rough rolling, the super-high pressure descaling andfinish rolling are carried out. In this case, the conditions for such asuper-high pressure descaling are required to have a jetting pressure onthe surface of the steel sheet: not less than 25 kgf/cm² and a liquidquantity density: not less than 0.002 liter/cm² as shown in FIG. 1 and atime within 5 seconds till the finish rolling is started after thedescaling as shown in FIG. 2 in order to control the average scalethickness to not less than 4 μm.

Here, the liquid quantity density is represented by a total liquid(water) quantity charged in the descaling per unit area of the steelsheet and determined by the following equation:

    W=Q·t/A                                           (1)

where

W: liquid quantity density (liter/cm²)

Q: discharging quantity (liter/sec)

t: time retaining the steel sheet under spraying (sec)

A: spraying area jetted on the steel sheet (cm²).

Moreover, the spraying area A jetted on the steel sheet (cm²) and thetime t retaining the steel sheet under spraying (sec) are determined bythe following equation using a steel sheet velocity v (cm/sec), spraynozzle widening angle x (degree) and distance H from the spray nozzle tothe steel sheet (cm).

When a shape of the spraying area A jetted on the steel sheet (cm²) is acircle having a radius r,

    A=πr.sup.2                                              (2)

    t=2r/v                                                     (3)

Substituting for the equations (2) and (3),

    W=2Q/(πr·v)                                    (4)

Further,

    r=H·tan (x/2)                                     (5),

so that adding the equation (5) to the equation (4),

    W=2Q/(π·H·tan (x/2)·v)       (6)

That is, the liquid quantity density W can be adjusted by thedischarging quantity Q, steel sheet velocity v, spray nozzle wideningangle x and distance H from the spray nozzle to the steel sheet.

These conclusions are obtained by the following experiment. Thecomposition of steel to be used in the experiment is 0.03 wt % C-0.01 wt% Si-0.12 wt % Mn-0.004 wt % P-0.007 wt % S-0.05 wt % Al-0.003 wt % N.Furthermore, the slab thickness: 260 mm, the slab heating temperature:1150° C., the rough rolling is 7 pass, the complete temperature:930°-970° C. (Ar₃ =870° C.), the sheet bar thickness is 40 mm, thefinish rolling is 7 pass, the finish temperature: 875° C., the finishsheet thickness: 3.5 mm, and the coiling temperature is 610° C.

Furthermore, the scale thickness of the hot rolled steel sheet iscalculated from weight difference before and after the pickling when asteel sheet punched out to 36 mmφ is descaled by pickling with 20%hydrochloric acid (50° C.) and a specific gravity of scale is 5.2 g/cm³.The positions of scale thickness to be measured are the vicinity of thecenter in the longitudinal direction of each steel band and 1/4 thereofin the widthwise direction, and the scale thickness is an average ofmeasured values at 5 positions.

Moreover, the jetting pressure p on the surface of the steel sheet inthe descaling can generally be measured by the following equation fromthe discharging pressure P and quantity Q from the nozzle and thedistance H between the surface of the steel sheet and the nozzle (see"Tetsu-to-Hagane", 1991, vol. 77, No. 9, page 1454, equation (4)):

    p=5.64PQ/H.sup.2                                           (7)

where

p: jetting pressure on the surface of the steel sheet (MPa)

P: discharging pressure (MPa)

Q: discharging quantity (liter/sec)

H: distance between steel sheet surface and nozzle (cm)

Although the mechanism of influencing the super-high pressure descalingconditions and the time until the start of finish rolling after thedescaling upon the final scale thickness is not entirely clear in theinvention, it is considered that as the jetting pressure is assuper-high as 25 kg/cm², the unevenness of the surface layer isdisappeared and smoothened to restrain the local formation of thickscale on the concave portion, and as the water quantity density exceeds0.002 liter/cm², only the extreme surface layer is effectively cooled toconsiderably suppress the scale formation in about 5 seconds after thedescaling. Further, it is considered that as a result of particularlycontrolling the rough rolling conditions in the invention, the steelsheet surface at the middle stage of the hot rolling is low in theroughness, then brings about the effect of controlling the growth ofscale in the thickness direction.

Incidentally, the jetting pressure in the conventional high-pressuredescaling is about 1.0-4.0 kgf/cm². In the invention, it seems thatcharacteristic action and effect, which have never been expected in theconventional technique, are developed by adopting the super-highpressure corresponding to about 10 times of the above value.

In the finish rolling followed to the super-high pressure descaling, itis then required to coil below 700° C. at a rolling reduction of notless than 80% under condition that the rolling completion temperature isabove Ar₃ point.

Because, when the rolling is carried out at lower than Ar₃ point, therolled structure remains, or unfavorable structure is formed to degradethe properties, while when the rolling reduction of the finish rollingis less than 80%, the malleability of scale through rolling isinsufficient and hence the thin scale is not attained. And also, whenthe coiling temperature exceeds 700° C., not only the growth of scale isconspicuous at the coil end portion after the coiling but also thecrystal grain is abnormally coarsened to cause inconveniences such asthe degradation of the properties and the like.

EXAMPLES Example 1

A slab of steel containing C: 0.0025 wt %, Si: 0.01 wt %, Mn: 0.15 wt %,P: 0.009 wt %, S: 0.006 wt %, sol.Al: 0.05 wt % and N: 0.0027 wt % washeated to 1150° C., subjected to rough rolling at various temperaturesshown in Table 1 to form a sheet bar of 35 mm, which was finish rolledat a reduction of 90% to a thickness of 3.5 mm and completed at a finishrolling temperature of 910° C. (Ar₃ =910° C.). The coiling temperaturewas 550° C. In this case, the descaling conditions and the time up tothe start of finish rolling after the descaling were varied as shown inTable 1. Moreover, the water discharging quantity Q, steel sheetvelocity v, spray nozzle widening angle x and distance from spray nozzleto steel sheet H in the descaling were 1 liter/sec, 40 m/min, 40 degreeand 10 cm as basic conditions, respectively. In order to obtain givenliquid quantity density and jetting pressure, the discharging pressureP, water discharging quantity Q, steel sheet velocity v and distancefrom spray nozzle to steel sheet H were properly changed according tothe equations (6) and (7).

After the resulting hot rolled steel sheet was cooled to roomtemperature, the average thickness of the scale was measured in thesimilar manner as described in FIGS. 1 and 2, while the surfaceroughness Ra was measured at a position corresponding to 1/4 of thewidthwise direction near to the center of the longitudinal direction ofeach steel sheet by every 5 positions in the longitudinal direction andwidthwise direction to determine a surface roughness Ra from theirweighted average. Furthermore, the pickling time was a time until thescale was completely peeled with 20% hydrochloric acid (50° C.). Andalso, it was cold rolled (rolling reduction 75%, thickness 0.7 mm) andannealed (continuous annealing at 800° C. for 60 seconds) and then theproperties were measured. These results were shown in Table 1 together.

As seen from Table 1, the hot rolled steel sheets according to theinvention had a thin scale having an average scale thickness of not morethan 4 μm and a surface roughness Ra of not more than 0.8 μm and weregood in not only the pickling property but also the properties aftercold rolling.

                                      TABLE 1                                     __________________________________________________________________________    Temper-                                                                       ature                                                                         in                                                                            comple-                                                                            Descaling conditions                                                                         Time until                                                                              Properties of scale                                                                     Mechanical properties of              tion of        Water                                                                              start of                                                                           Surface                                                                            Average   cold rolled steel sheet               rough                                                                              Discharge                                                                          Jetting                                                                            quantity                                                                           finish                                                                             roughness                                                                          scale                                                                              Pickling       Elonga-                     rolling                                                                            pressure                                                                           pressure                                                                           density                                                                            rolling                                                                            Ra   thickness                                                                          time YS   TS   tion                                                                              Average                 (°C.)                                                                       (kgf/cm.sup.2)                                                                     (kgf/cm.sup.2)                                                                     (l/cm.sup.2)                                                                       (second)                                                                           (μm)                                                                            (μm0                                                                            (second)                                                                           (kgf/mm.sup.2)                                                                     (kgf/mm.sup.2)                                                                     (%) r-value                                                                           Remarks             __________________________________________________________________________    980  550  29.5 0.0024                                                                             2.3  0.62 3.5  35   16.3 30.7 49.1                                                                              1.78                                                                              Inven-              995  620  33.3 0.0024                                                                             3.7  0.71 3.1  30   17.1 30.5 48.7                                                                              1.75                                                                              tion                965  600  32.2 0.0024                                                                             4.8  0.53 2.8  27   17.0 30.5 48.6                                                                              1.71                                                                              Example             940  500  26.8 0.0024                                                                             4.8  0.87 4.8  51   16.5 30.6 48.8                                                                              1.80                                                                              Compar-             1020 530  28.4 0.0024                                                                             5.0  1.03 5.0  63   16.7 30.5 49.5                                                                              1.83                                                                              ative               963  465  24.9 0.0025                                                                             4.2  1.25 7.3  108  16.5 30.4 48.9                                                                              1.75                                                                              Example             985  590  31.7 0.0018                                                                             2.9  0.91 5.5  72   17.0 30.3 48.8                                                                              1.78                    970  620  33.3 0.0020                                                                             3.3  0.78 3.5  45   16.8 31.0 49.5                                                                              1.78                                                                              Inven-                                                                        tion                                                                          Example             970  600  32.2 0.0023                                                                             5.3  0.75 6.9  103  17.2 30.7 49.1                                                                              1.81                                                                              Compar-                                                                       ative                                                                         Example             __________________________________________________________________________

Example 2

A slab of steel containing C: 0.08 wt %, Si: 0.01 wt %, Mn: 0.51 wt %,P: 0.011 wt %, S: 0.008 wt %, sol.Al: 0.04 wt % and N: 0.004 wt % washeated to 1200° C., subjected to rough rolling at various temperaturesshown in Table 2 to form a sheet bar of 35 mm, which was then subjectedto finish rolling at a reduction of 92% to a thickness of 2.8 mm and thefinish rolling was completed at 875° C. (Ar₃ point=850° C.). The coilingtemperature was 610° C. In this case, the descaling conditions and thetime until the start of the finish rolling after the descaling werechanged as shown in Table 2.

After the resulting hot rolled steel sheet was cooled to roomtemperature, the scale thickness and surface roughness Ra (μm) weremeasured in the same manner as in Example 1. The results were also shownin Table 2. In this case, the pickling time was a time until the scalewas completely peeled with 20% hydrochloric acid (50° C.).

As seen from Table 2, the hot rolled steel sheets produced according tothe invention had an average scale thickness of not more than 4 μm and asurface roughness Ra of not more than 0.8 μm and were good in thepickling property.

                                      TABLE 2                                     __________________________________________________________________________    Temper-                                                                       ature                                                                         in                               Properties                                   comple-                                                                            Descaling conditions                                                                           Time until of scale   Mechanical properties of          tion of         Water start of                                                                           Surface                                                                             Average    hot rolled steel sheet            rough                                                                              Discharge                                                                           Jetting                                                                            quantity                                                                            finish                                                                             roughness                                                                           scale                                                                              Pickling        Elonga-                 rolling                                                                            pressure                                                                            pressure                                                                           density                                                                             rolling                                                                            Ra    thickness                                                                          time  YS   TS   tion                    (°C.)                                                                       (kgf/cm.sup.2)                                                                      (kgf/cm.sup.2)                                                                     (l/cm.sup.2)                                                                        (second)                                                                           (μm)                                                                             (μm)                                                                            (second)                                                                            (kgf/mm.sup.2)                                                                     (kgf/mm.sup.2)                                                                     (%) Remarks             __________________________________________________________________________    945  550   29.5 0.0024                                                                              2.3  0.61  3.5  36    24.7 36.5 47.5                                                                              Inven-              940  620   33.3 0.0024                                                                              3.6  0.72  3.2  31    25.2 37.5 47.1                                                                              tion                925  600   32.2 0.0024                                                                              4.9  0.48  2.9  25    26.1 37.3 46.3                                                                              Example             890  500   26.8 0.0024                                                                              4.8  0.89  4.8  53    25.7 36.4 45.9                                                                              Compar-             980  530   28.4 0.0025                                                                              5.3  1.11  5.1  64    24.3 37.0 46.3                                                                              ative               945  460   24.7 0.0025                                                                              4.3  1.24  7.5  111   25.5 36.9 47.5                                                                              Example             955  590   31.7 0.0017                                                                              2.9  0.90  5.4  73    25.6 37.0 47.1                    950  620   33.3 0.0021                                                                              3.4  0.78  3.4  45    24.7 38.0 46.9                                                                              Invention                                                                     Example             930  600   32.2 0.0023                                                                              5.5  0.74  6.8  105   26.4 37.7 46.2                                                                              Compar-                                                                       ative                                                                         Example             940  680   63.4 0.0028                                                                              3.2  0.62  2.7  24    26.5 39.5 47.2                                                                              Inven-                                                                        tion                                                                          Example             __________________________________________________________________________

Example 3

Each of steel slabs having a chemical composition shown in Table 3 washeated to 1200° C., rough rolled to a sheet bar of 35 mm, descaled, andsubjected to finish rolling at a reduction of 90% to a thickness of 3.5mm. The production conditions were summarized in Table 4.

After the resulting hot rolled steel sheet was cooled to roomtemperature, the scale thickness, surface roughness and pickling timewere measured in the same manner as in Example 1. The results were alsoshown in Table 4.

As seen from Tables 3 and 4, the hot rolled steel sheets producedaccording to the invention had an average scale thickness of not morethan 4 μm and a surface roughness not more than 0.8 μm and were good inthe pickling property.

                                      TABLE 3                                     __________________________________________________________________________    Chemical composition (wt %)                                                   No                                                                              C   Si  Mn P   S  sol Al                                                                            N   Ti  Nb B                                          __________________________________________________________________________    1 0.0027                                                                            0.46                                                                              1.20                                                                             0.060                                                                             0.006                                                                            0.05                                                                              0.0028                                                                            0.05                                                                              -- 0.0026                                     2 0.0025                                                                            0.02                                                                              0.16                                                                             0.008                                                                             0.008                                                                            0.05                                                                              0.0025                                                                            0.06                                                                              -- --                                         3 0.0021                                                                            0.02                                                                              0.17                                                                             0.007                                                                             0.009                                                                            0.04                                                                              0.0031                                                                            0.05                                                                              0.006                                                                            --                                         4 0.0026                                                                            0.01                                                                              0.12                                                                             0.009                                                                             0.012                                                                            0.06                                                                              0.0032                                                                            --  0.04                                                                             --                                         5 0.0300                                                                            0.01                                                                              0.15                                                                             0.008                                                                             0.014                                                                            0.04                                                                              0.0027                                                                            --  -- 0.0021                                     6 0.0027                                                                            0.01                                                                              0.14                                                                             0.006                                                                             0.008                                                                            0.07                                                                              0.0026                                                                            0.06                                                                              -- 0.0011                                     7 0.0021                                                                            0.02                                                                              0.16                                                                             0.008                                                                             0.006                                                                            0.06                                                                              0.0028                                                                            --  0.03                                                                             0.0008                                     8 0.0019                                                                            0.01                                                                              0.15                                                                             0.008                                                                             0.008                                                                            0.05                                                                              0.0029                                                                            0.04                                                                              0.008                                                                            0.0012                                     __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Temper-                                                                       ature                                           Properties                    in      Descaling conditions                                                                           Time until                                                                         Finish rolling    of scale                      comple-            Water start of                                                                           conditions                                                                             Coiling                                                                           Surface                                                                            Average                         tion of                                                                             Discharge                                                                           Jetting                                                                            quantity                                                                            finish                                                                             Complete temper-                                                                           roughness                                                                          scale                                                                              Pickling                   rough pressure                                                                            pressure                                                                           density                                                                             rolling                                                                            temper-                                                                            Ar.sub.3                                                                          ature                                                                             Ra   thickness                                                                          time                     No                                                                              rolling (°C.)                                                                (kgf/cm.sup.2)                                                                      (kgf/cm.sup.2)                                                                     (l/cm.sup.2)                                                                        (second)                                                                           ature (°C.)                                                                 (°C.)                                                                      (°C.)                                                                      (μm)                                                                            (μm)                                                                            (second)                                                                           Remarks             __________________________________________________________________________    1 950   550   27.9 0.0024                                                                              4.1  910  880 630 0.62 3.9  46   Inven-              2 970   540   27.1 0.0026                                                                              4.4  920  900 670 0.74 3.8  35   tion                3 960   600   30.1 0.0025                                                                              4.2  915  905 550 0.75 3.8  34   Exam-               4 980   620   31.1 0.0021                                                                              4.5  915  905 660 0.78 3.7  36   ple                 5 930   610   30.6 0.0021                                                                              3.8  870  860 640 0.66 3.2  30                       6 950   580   29.1 0.0026                                                                              3.9  910  890 650 0.72 3.8  35                       7 970   590   29.6 0.0021                                                                              4.2  915  900 630 0.70 3.7  32                       8 980   610   30.6 0.0027                                                                              3.6  925  910 600 0.63 3.6  33                       __________________________________________________________________________

INDUSTRIAL APPLICABILITY

As mentioned above, the hot rolled steel sheets according to theinvention are thin in the scale thickness, good in the adhesion propertyand very less in the peeling in applications that they are applied toworking as-rolled (at a state of mill scale) and are good in thepickling property and have an excellent surface quality in applicationsused after the pickling.

According to the production method of the invention, the above hotrolled steel sheets can be produced very effectively by applying thesuper-high pressure descaling in the hot rolling step.

Therefore, the invention largely contributes to the productivity andeconomy of various products such as hot rolled steel sheets, cold rolledsteel sheets using the hot rolled steel sheet as a starting material,surface-treated steel sheets and the like.

We claim:
 1. A hot rolled steel sheet comprising C: 0.001-0.20 wt %, Si:0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %, S: notmore than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than 0.020 wt %and the balance being Fe and inevitable impurities, and having anaverage surface scale thickness of not more than 4 μm and a surfaceroughness (Ra) of not more than 0.8 μm developed after rough rolling ata completing temperature of (Ar₃ point+100° C.)-(Ar₃ point+50° C.) andsuper-high pressure descaling at a liquid quantity density of not lessthan 0.002 liter/cm² under a jetting pressure of not less than 25kgf/cm² and then subsequent finish rolling.
 2. A hot rolled steel sheetcomprising C: 0.001-0.20 wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P:not more than 0.05 wt %, S: not more than 0.05 wt %, sol.Al: 0.01-0.10wt %, N: not more than 0.020 wt %, one or two of Ti: not more than 0.10wt % and Nb: not more than 0.10 wt % and the balance being Fe andinevitable impurities, and having an average surface scale thickness ofnot more than 4 μm and a surface roughness (Ra) of not more than 0.8 μmdeveloped after rough rolling at a completing temperature of (Ar₃point+100° C.)-(Ar₃ point+50° C.) and super-high pressure descaling at aliquid quantity density of not less than 0.002 liter/cm² under a jettingpressure of not less than 25 kgf/cm² and then subsequent finish rolling.3. A hot rolled steel sheet comprising C: 0.001-0.20 wt %, Si: 0.01-0.50wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %, S: not more than0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than 0.020 wt %, B: notmore than 0.0100 wt % and the balance being Fe and inevitableimpurities, and having an average surface scale thickness of not morethan 4 μm and a surface roughness (Ra) of not more than 0.8 μm developedafter rough rolling at a completing temperature of (Ar₃ point+100°C.)-(Ar₃ point+50° C.) and super-high pressure descaling at a liquidquantity density of not less than 0.002 liter/cm² under a jettingpressure of not less than 25 kgf/cm² and then subsequent finish rolling.4. A hot rolled steel sheet comprising C: 0.001-0.20 wt %, Si: 0.01-0.50wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %, S: not more than0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than 0.020 wt %, one ortwo of Ti: not more than 0.10 wt % and Nb: not more than 0.10 wt %, B:not more than 0.0100 wt % and the balance being Fe and inevitableimpurities, and having an average surface scale thickness of not morethan 4 μm and a surface roughness (Ra) of not more than 0.8 μm developedafter rough rolling at a completing temperature of (Ar₃ point+100°C.)-(Ar₃ point+50° C.) and super-high pressure descaling at a liquidquantity density of not less than 0.002 liter/cm² under a jettingpressure of not less than 25 kgf/cm² and then subsequent finish rolling.5. A method of producing a hot rolled steel sheet, which comprisesheating a starting material of steel comprising C: 0.001-0.20 wt %, Si:0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P: not more than 0.05 wt %, S: notmore than 0.05 wt %, sol.Al: 0.01-0.10 wt %, N: not more than 0.020 wt %and the balance being Fe and inevitable impurities to not lower than Ac₃point, completing rough rolling within a temperature range of (Ar₃point+100° C.)˜(Ar₃ point+50° C.), conducting super-high pressuredescaling under conditions satisfying a jetting pressure of not lessthan 25 kgf/cm² and a liquid quantity density of not less than 0.002liter/cm², starting finish rolling at a rolling reduction of not lessthan 80% above Ar₃ point of rolling complete temperature within 5seconds and coiling up below 700° C.
 6. A method of producing a hotrolled steel sheet, which comprises heating a starting material of steelcomprising C: 0.001-0.20 wt %, Si: 0.01-0.50 wt %, Mn: 0.05-2.0 wt %, P:not more than 0.05 wt %, S: not more than 0.05 wt %, sol.Al: 0.01-0.10wt %, N: not more than 0.020 wt %, one or more of Ti: not more than 0.10wt %, Nb: not more than 0.10 wt % and B: not more than 0.0100 wt % andthe balance being Fe and inevitable impurities to not lower than Ac₃point, completing rough rolling within a temperature range of (Ar₃point+100° C.)˜(Ar₃ point+50° C.), conducting super-high pressuredescaling under conditions satisfying a jetting pressure of not lessthan 25 kgf/cm² and a liquid quantity density of not less than 0.002liter/cm², starting finish rolling at a rolling reduction of not lessthan 80% above Ar₃ point of rolling complete temperature within 5seconds and coiling up below 700° C.